1. Technical Field
This invention generally relates to the field of galvanic cathodic protection of steel embedded in concrete structures, and is particularly concerned with the performance of embedded sacrificial anodes, such as zinc, aluminum, and alloys thereof.
2. Background Art
The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to “passivate” such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt over a number of years results in the concrete becoming contaminated with chloride ions. Salt is commonly introduced in the form of seawater, set accelerators, or deicing salt.
When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contamination, it destroys the ability of the concrete to keep the steel in a passive, non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of the steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate. In a recent report to Congress, the Federal Highway Administration reported that of the nation's 577,000 bridges, 266,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.
Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques, such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt-contaminated concrete.
Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several different types of anodes have evolved for specific circumstances and different types of structures.
The most commonly used type of cathodic protection system is impressed current cathodic protection (ICCP), which is characterized by the use of inert anodes, such as carbon, titanium suboxide, and most commonly, catalyzed titanium. ICCP also requires the use of an auxiliary power supply to cause protective current to flow through the circuit, along with attendant wiring and electrical conduit. This type of cathodic protection has been generally successful, but problems have been reported with reliability and maintenance of the power supply. Problems have also been reported related to the durability of the anode itself, as well as the concrete immediately adjacent to the anode, since one of the products of reaction at an inert anode is acid (H+). Acid attacks the integrity of the cement paste phase within concrete. Finally, the complexity of ICCP systems requires additional monitoring and maintenance, which results in additional operating costs.
A second type of cathodic protection, known as galvanic cathodic protection (GCP), offers certain important advantages over ICCP. GCP uses sacrificial anodes, such as zinc and aluminum, and alloys thereof, which have inherently negative electrochemical potentials. When such anodes are used, protective current flows in the circuit without need for an external power supply since the reactions that occur are thermodynamically favored. GCP therefore requires no rectifier, external wiring or conduit. This simplicity increases reliability and reduces initial cost, as well as costs associated with long term monitoring and maintenance. Also, the use of GCP to protect high-strength prestressed steel from corrosion is considered inherently safe from the standpoint of hydrogen embrittlement. Recognizing these advantages, the Federal Highway Administration issued a Broad Agency Announcement (BAA) in 1992 for the study and development of sacrificial anode technology applied to reinforced and prestressed bridge components. As a result of this announcement and the technology that was developed because of this BAA, interest in GCP has greatly increased over the past few years.
In U.S. Pat. No. 6,022,469 by Page a method of galvanic cathodic protection is disclosed wherein a zinc or zinc alloy anode is surrounded by a mortar containing an agent to maintain a high pH in the mortar surrounding the anode. This agent, preferably lithium hydroxide (LiOH), serves to prevent passivation of the zinc anode and maintain the anode in an electrochemically active state. In this method, the zinc anode is electrically attached to the reinforcing steel causing protective current to flow and mitigating subsequent corrosion of the steel.
In U.S. Pat. No. 5,292,411 Bartholomew et al discloses a method of patching an eroded area of concrete comprising the use of a metal anode having an ionically conductive hydrogel attached to at least a portion of the anode. In this patent it is taught that the anode and the hydrogel are flexible and are conformed within the eroded area, the anode being in elongated foil form.
In U.S. Pat. No. 6,471,851 issued to Bennett on Oct. 29, 2002 the use of deliquescent or hygroscopic chemicals, collectively called “humectants” is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO3 and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742 B1, issued Apr. 17, 2001, Bennett discloses the use of LiNO3 and LiBr to enhance the performance of embedded discrete anodes. And finally, in U.S. Pat. No. 6,165,346, issued Dec. 26, 2000, Whitmore broadly claims the use of deliquescent chemicals to enhance the performance of the apparatus disclosed by Page in U.S. Pat. No. 6,022,469.
In U.S. Pat. No. 7,160,433 B2 issued Jan. 9, 2007, a method of cathodic protection of reinforcing steel is disclosed comprising a sacrificial anode embedded in an ionically conductive compressible matrix designed to absorb the expansive products of corrosion of the sacrificial anode metal.
In U.S. Pat. No. 6,572,760 B2, issued Jun. 3, 2003, Whitmore discloses the use of a deliquescent material bound into a porous anode body, which acts to maintain the anode electrochemically active, while providing room for the expansive products of corrosion. The same patent discloses several mechanical means of making electrical connection to the reinforcing steel within a hole drilled into the concrete covering material. Many of these means involve driven pins, impact tools, and other specialized techniques. These techniques are all relatively complex and difficult to perform.
Finally, in U.S. Pat. No. 6,193,857, issued Feb. 27, 2001, Davison, et al describes an anode assembly comprising a block of anode material cast around an elongated electrical connector (wire). Other claims disclose making contact between the elongated connector and the reinforcing steel by winding the connector around the reinforcing steel and twisting the ends of the connector together using a twisting tool.
The anodes described above and the means of connection disclosed have become the basis for commercial products designed to extend the life of patch repair and to cathodically protect reinforced concrete structures from corrosion. But some embodiments, such as the use of high pH to maintain the anode in an electrochemically active state as described by Page, result in protective current that is small and often inadequate to mitigate corrosion. Use of the chemicals such as lithium nitrate and lithium bromide, result in a higher current, but even this current is sometimes inadequate in cases of high chloride contamination and the presence of strong corrosion of the reinforcing steel.
Also, some of the chemicals used to maintain the zinc anode in an electrochemically active state render the corrosion products of zinc largely insoluble. In this case the expansive corrosion products apply stress to the surrounding concrete, and when this stress exceeds the tensile strength of the concrete, cracking of the concrete can occur. Although several potential solutions have been proposed, including the ionically compressible conductive matrix described in U.S. Pat. No. 7,160,433 B2, cracking remains a problem in some cases.
It would be of great benefit to increase the protective current higher than was previously possible using the prior art as described in the patent literature above. It would also be of benefit to overcome the problem of potential cracking of the overlaying concrete due to expansive corrosion products.